1. Field of the Invention
The present invention relates to a method for pretreating a surface of a semiconductor substrate which is a prestage for forming a thin film on the semiconductor substrate and, more particularly, to a method for removing a natural oxide film or a contaminant adhering to the surface of a semiconductor substrate.
2. Description of the Background Art
Since the characteristic of an electric device is strongly affected by the existence of an impurity introduced at an intentional or an unintentional accident during its manufacturing, it is necessary to hold the cleanliness of a manufacturing atmosphere at a high level throughout all processes. To that end, a high degree of cleaning and purification technique is used in a material, a method for forming a treatment atmosphere and the like.
The manufacturing processes for semiconductor devices are classified roughly into a thin film forming process and a circuit pattern forming process. The thin film forming process is further classified into various processes depending on the kind of film and its forming method, in which original or partially common cleaning technique has been developed. The pretreatment on a substrate before the formation of a thin film is an important and basic cleaning process common to all processes.
In the pretreatment processing, water washing, acid or alkali cleaning, chemical oxidation, dilute hydrofluoric acid treatment and the like are usually performed for the purposes of degreasing, and removing heavy metal and a natural oxide film.
Although these solution cleaning methods are widely adopted as an established process, a crucial problem is that a natural oxide film grows to some extent without exception especially when an active semiconductor surface or metal surface is exposed on the substrate because the substrate after treatment is surely exposed to the air during the time from the end of the treatment until the start of the thin film formation. Therefore, although the solution cleaning is effective in removing impurities of an organic substance, a heavy metal and the like, it cannot necessarily be means for obtaining a clean surface.
The growth of a natural oxide film has crucially a bad influence on the quality of a thin film in the later process of forming a thin film. The kinds of the above described thin film formation comprise, for example, epitaxial growth, formation of a refractory metal film on polysilicon (so called polycide structure), formation of wiring to make an electrical contact to a substrate, formation of an extremely thin insulating film and the like, and these processes will have an increased importance in the future as a high degree of integration is implemented.
Therefore, in order to form a thin film having well-controlled interface structure in a substrate, first it is most important to remove a natural oxide film growing on the semiconductor substrate surface, so that an excellent method for removing a natural oxide film is strongly demanded.
There is no excellent method for removing a natural oxide film, and the following technique is adopted at the present. That is, the technique is such that the substrate after solution cleaning is introduced to a thin film forming device and a natural oxide film is removed by sputter etching by means of plasma of an inert gas such as Ar or by gas etching by a hydrogen reducing method at a high temperature and, then, a thin film is formed continuously thereon. However, in the sputter etching by means of plasma of an inert gas such as Ar, the substrate is damaged. In addition, since a high temperature (usually more than 1,000.degree. C.) is needed in a high temperature hydrogen reducing method, heat shear droop of a PN junction and the like is caused, so that its application is limited.
Although as a recent tendency in manufacturing a device, one having a small crystal grain diameter, that is, an almost amorphous one is preferred as crystalline material such as polysilicon, the grain diameter is increased in the material such as polysilicon which was exposed to a high temperature so that the high temperature hydrogen reducing method has many restrictions also in this respect, as pointed out by E. Kinsbron, M. Sternheim, and R. Knoell, in Appl. Phys. Lett., Vol. 42, No. 9,835, 1 May (1983).
In addition, it is known that chlorine radicals excited by ultraviolet light can be used to clean silicon surfaces through native oxide (Extended Abstract of the 19th Conference on Solid State Devices and Materials, Tokyo, 1987, pp. 207-210). However, the cleaning effects were not sufficient.
The present invention was made to solve the above described problems and it is an object of the present invention to provide a method for pretreating a semiconductor substrate surface by which a natural oxide film or contaminant attached onto the semiconductor substrate surface can be removed at a sufficiently low temperature without damaging the semiconductor substrate surface.
The present invention is most important in that a reaction gas to be used does not absorb irradiation light and, therefore, photochemical excitation is not performed in a vapor state, so that there are some cases where a remarkable light irradiation effect can be attained if the surface of the substrate is irradiated with light and the substrate is heated appropriately even in a reaction system in which the light irradiation is regarded as invalid. Conventionally, many attempts have been reported in which the semiconductor substrate surface just before the film formation is pretreated at a low temperature with no damage, using the photochemical reaction as intended by the present invention. However, all of them had the fixed idea that it was necessary for the reaction gas to absorb the irradiation light as a photochemical process, so that there were many restrictions in coupling the reaction gas and the irradiation light source. In fact, since originally the photochemistry of gas which was one theoretical background of photoexcitation processing was mainly directed to the photochemistry in the vapor state, it was essential for the reaction gas to absorb the irradiation light to achieve a photochemical process. In addition, in applying the photochemistry to the semiconductor device manufacturing process, the influence of temperature on the treated substrate serving as absorbent was ignored when the photochemistry in the state absorbed into the solid surface was considered. Although the substrate to be treated was heated in many cases in a photoexcitation CVD method and the like, this heating was performed in order to improve a film quality such as minuteness of a deposited film, so that the photochemical reaction itself occurred even if the heating was not performed. In other words, it is understood that the influence of heat energy on the photochemical process was not considered at all in a conventional system of the photochemical reaction or photoexcitation process. The present invention aimed at this respect and various experiments were performed. As a result, it was found that there were some cases in which excitation similar to the photochemical excitation occurred when even a reaction gas which did not absorb the irradiation light in the vapor state was absorbed in the substrate of a high temperature and thus the present invention was completed.
More specifically, the present invention provides a method for removing a natural oxide film or contaminant attached on the surface of a semiconductor substrate and the inventive method comprises the steps of placing the above described semiconductor substrate in a chamber; introducing to the chamber an inert gas capable of reacting with the above described natural oxide film or contaminant; heating the above described semiconductor substrate to a temperature within the range of 200.degree..about.700.degree. C.; and irradiating the above described heated semiconductor substrate with the light having a wavelength which causes the photochemical reaction of the reaction gas introduced into the above described chamber with the natural oxide film or contaminant adhering on the semiconductor substrate surface at a temperature within the range of 200.degree..about.700.degree. C. while the above described semiconductor substrate is heated.
The reaction gas used in the present invention is generally hydrogen chloride gas, hydrogen gas, chlorine gas or the like, and particularly hydrogen chloride gas is preferably used. The semiconductor substrate is heated to a temperature within the range of 200.about.700.degree. C. Although a detailed description is made with data shown hereinafter, a temperature not within the range of 200.degree..about.700.degree. C. is not preferable because a treatment speed of the natural oxide film is slow below 200.degree. C. and the above mentioned heat shear droop and undesired increase in the grain diameter in the crystalline material are caused above 700.degree. C. while the treatment speed of a natural oxide film becomes faster.
If the irradiation light has a wavelength causing the photochemical reaction of the reaction gas introduced in the chamber with a natural oxide film or contaminant on the semiconductor substrate surface at a temperature within the range of 200.degree..about.700.degree. C. while the semiconductor substrate is heated, any light can be used. Particularly, the irradiation light from a low pressure mercury lamp, a high pressure mercury lamp, mercury-xenon lamp, excimer laser or the like is preferable.
Although the irradiating direction of the above described light is preferably toward the semiconductor substrate surface, a direction parallel to the semiconductor substrate surface may be adopted. When the light is irradiated in a direction toward the semiconductor substrate surface, the low pressure mercury lamp, the high pressure mercury lamp and the mercury-xenon lamp of the above described light sources are preferably used as a light source.
When the light is irradiated in a direction parallel to the semiconductor substrate surface, particularly the excimer laser light is preferably used.
The pressure in the atmosphere of the reaction gas is preferably selected to be within the range of atmospheric pressure to 0.1 Torr.
In addition, for the same purpose, the light may be used as means for heating the semiconductor substrate to a temperature within the range of 200.degree..about.700.degree. C. The light source for heating the semiconductor substrate may be the same as that irradiating the reaction gas. In this case, an argon-arc lamp and a xenon-mercury lamp are preferably used. Furthermore, the light for heating the semiconductor substrate may be infrared rays and the light with which the reaction gas is irradiated may be ultraviolet rays.
As described above, the present invention uses both light and heat to remove a natural oxide film or contaminant adhering on the surface of a semiconductor substrate using a reaction gas. When a HCl gas is used as a reaction gas, the natural oxide film is removed from the substrate in accordance with the following reaction formula: EQU SiO.sub.2 +4HCl.fwdarw.SiCl.sub.4 +2H.sub.2 O
The present invention makes use of the fact that this reaction is promoted by a synergistic effect of light and heat energy. Therefore, even if a reaction gas which can not be activated only by light is used, a natural oxide film or contaminant can be removed by the reaction gas. In addition, even when the natural oxide film or contaminant can be removed only under a high temperature, the natural oxide film or contaminant can be removed at a lower temperature with the help of the light energy.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.